Synthesis of PbTiO3 ceramics using mechanical alloying and solid state sintering

The pressure-less sintering behavior of PbTiO₃ powders synthesized by mechanical alloying TiO₂ and PbO was investigated using dilatometry and Rietveld refinements of X-ray diffraction patterns. As-synthesized, the powders are nanocrystalline with a mean particle size of 20 nm. Pressure-less sintering in the range 500-1050°C gives single phase ceramics with densities of 85-90% and crystallite sizes in the range 80-400 nm. Cracking due to the paraelectric-ferroelectric phase transition was not observed in samples sintered below 700°C due to the small crystallite size whereas macroscopic cracks formed in samples sintered above 700°C. Rietveld analysis indicates the formation of Pb vacancies in samples sintered and held for 24 h at intermediate temperatures (600-1000°C) which gives some insight into the mechanism of Pb loss and second phase formation in this system.     

Phosphorus-based SAHA analogues as histone deacetylase inhibitors

[structure: see text] Three analogues of suberoyl anilide hydroxamic acid (SAHA) with phosphorus metal-chelating functionalities were synthesized as inhibitors of histone deacetylases (HDACs). The compounds showed weak activity for HeLa nuclear extracts (IC(50) = 0.57-6.1 mM), HDAC8 (IC(50) = 0.28-0.41 mM), and histone-deacetylase-like protein (HDLP, IC(50) = 0.33-1.9 mM), suggesting that the transition state of HDAC is not analogous to zinc proteases. Antiproliferative activity against A2780 cancer cells (IC(50) = 0.11-0.12 mM), comparable to SAHA (0.15 mM), was observed.     

On the effect of covalently appended quinolones on termini of DNA duplexes

Quinolones are gyrase inhibitors that are widely used as antibiotics in the clinic. When covalently attached to oligonucleotides as 5'-acylamido substituents, quinolones were found to stabilize duplexes of oligonucleotides against thermal denaturation. For short duplexes, such as qu-T*GCGCA, where qu is a quinolone residue and T is a 5'-amino-5'-deoxythymidine residue, an increase in the UV melting point of up to 27.8 degrees C was measured. The stabilizing effect was demonstrated for all quinolones tested, namely nalidixic acid, oxolinic acid, pipemidic acid, cinoxacin, norfloxacin, and ofloxacin. The three-dimensional structure of (oa-T*GCGCA)2, where oa is an oxolinic acid residue, was solved by two-dimensional NMR spectroscopy and restrained molecular dynamics. In this complex, the oxolinic acid residues disrupt the terminal T1:A6 base pairs and stack on the G2:C5 base pairs. The displaced adenosine residues bind in the minor groove of the core duplex, while the thymidine residues pack against the oxolinic acid residues. The "molecular cap" thus formed fits tightly on the G:C base pairs, resulting in increased base-pairing fidelity, as demonstrated in UV melting experiments with the sequence oa-T*GGTTGAC and target strands containing a mismatched nucleobase. The structure of the "molecular cap" with its disrupted terminal base pair may also be helpful for modeling how quinolones block re-ligation of DNA strands in the active site of gyrases.     

Tertiary contact formation in alpha-synuclein probed by electron transfer

To explore tertiary contact formation in alpha-synuclein, a natively unfolded protein implicated in Parkinson's disease, we have measured the rates of reaction between a powerful electron donor, the tryptophan (W) triplet excited state, and an acceptor, 3-nitro-tyrosine (Y(NO2)) in six different variants, probing loop sizes between 15 and 132 residues. Electron transfer rates decrease with loop size with the fastest contact time of 140 ns for the N-terminal pair and the slowest of 1.2 mus for the N- to C-terminal pair. Diffusion coefficients ranging from approximately 2 x 10-6 to approximately 10-5 cm2 s-1 were extracted from simultaneous fits of the W to Y(NO2) electron (triplet excited state) and energy transfer (singlet excited state) kinetics.     

The interaction of water and dibromine in the gas phase: an investigation of the complex H(2)O...Br(2) by rotational spectroscopy and ab initio calculations

The ground-state rotational spectra of eight isotopomers of a complex formed by water and dibromine in the gas phase were observed by pulsed-jet, Fourier transform microwave spectroscopy. The spectroscopic constants B(0), C(0), delta(J), delta(JK), chi(aa)(Br(x)) (x=i for inner, o for outer), [chi(bb)(Br(x))-chi(cc)(Br(x))] and M(bb)(Br(x)) were determined for H(2)O...(79)Br(79)Br, H(2)O...(81)Br(79)Br, H(2)O...(79)Br(81)Br, H(2)O...(81)Br(81)Br, D(2)O...(79)Br(81)Br and D(2)O...(81)Br(81)Br. For the isotopomers HDO...(79)Br(81)Br and HDO...(81)Br(81)Br, only (B(0) + C(0))/2, delta(J), the chi(aa)(Br(x)) and M(bb)(Br(x)) were determinable. The spectroscopic constants were interpreted on the basis of several models of the complex to give information about its geometry, binding strength and the extent of electronic rearrangement on complex formation. The molecule H(2)O...Br(2) has C(s) symmetry with a pyramidal configuration at O. The zero-point effective quantities r(O...Br(i))=2.8506(1) A and phi(0)=46.8(1), where phi is the angle between the C(2) axis of H(2)O and the O...Br-Br internuclear axis, were obtained under the assumption of monomer geometries unchanged by complexation. Ab initio calculations, carried out at the aug-cc-pVDZ/MP2 level of theory, gave the equilibrium values r(e)(O...Br(i))=2.7908 A and phi(e)=45.7 degrees and confirmed the collinearity of the O...Br-Br nuclei. The potential energy function V(phi), also determined ab initio, showed that the wavenumber required for inversion of the configuration at O in the zero-point state is only 9 cm(-1). By interpreting the Br nuclear quadrupole coupling constants, the fractions delta(O-->Br(i))=0.004(5) and delta (Br(i)-->Br(o))=0.050(2) of an electron were determined to be transferred from O to Br(i) and Br(i) to Br(o), respectively, when the complex is formed. The complex is relatively weak, as indicated by the small value k(sigma)=9.8(2) N m(-1) of the intermolecular stretching force constant obtained from delta(J). A comparison of the properties, similarly determined, of H(2)O...F(2), H(2)O...Cl(2), H(2)O...Br(2), H(2)O...BrCl, H(2)O...ClF and H(2)O...ICl is presented.     

Pseudooctahedral complexes of vanadium(III): electronic structure investigation by magnetic and electronic spectroscopy

A variety of physical methods has been used to probe the non-Kramers, S = 1, V(III) ion in two types of pseudooctahedral complexes: V(acac)(3), where acac = anion of 2,4-pentanedione, and VX(3)(thf)(3), where thf = tetrahydrofuran and X = Cl and Br. These methods include tunable frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 95-700 GHz and fields up to 25 T) in conjunction with electronic absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD (VTVH-MCD) spectroscopies. Variable-temperature magnetic susceptibility and field-dependent magnetization measurements were also performed. All measurements were conducted on complexes in the solid state (powder or mull samples). The field versus sub-THz wave quantum energy dependence of observed HFEPR resonances yielded the following spin Hamiltonian parameters for V(acac)(3): D = +7.470(1) cm(-1); E = +1.916(1) cm(-1); g(x) = 1.833(4); g(y) = 1.72(2); g(z) = 2.03(2). For VCl(3)(thf)(3), HFEPR detected a single zero-field transition at 15.8 cm(-1) (474 GHz), which was insufficient to determine the complete set of spin Hamiltonian parameters. For VBr(3)(thf)(3), however, a particularly rich data set was obtained using tunable-frequency HFEPR, and analysis of this data set gave the folowing: D = -16.162(6) cm(-1); E = -3.694(4) cm(-1); g(x) = 1.86(1); g(y) = 1.90(1); g(z) = 1.710(4). Analysis of the VTVH-MCD data gave spin Hamiltonian parameters in good agreement with those determined by HFEPR for both V(acac)(3) and VBr(3)(thf)(3) and in rough agreement with the estimate for VCl(3)(thf)(3) (D approximately 10 cm(-1), |E/D| approximately 0.18), together with the finding that the value of D is negative for both thf complexes. The electronic structures of these V(III) complexes are discussed in terms of their molecular structures and the electronic transitions observed by electronic absorption and MCD spectroscopies.     

Superexcited state reconstruction of HCl using photoelectron and photoion imaging

The velocity-map imaging technique was used to record photoelectron and photofragment ion images of HCl following two-photon excitation of the E Sigma(+)(0+), V 1Sigma(+)(0+) (nu=9,10,11) states and subsequent ionization. The images allowed us to determine the branching ratios between autoionization and dissociation channels for the different intermediate states. These branching ratios can be explained on the basis of intermediate state electron configurations, since the configuration largely prohibits direct ionization in a one-electron process, and competition between autoionization and dissociation into H* (n=2)+Cl and H+Cl*(4s,4p,3d) is observed. From a fit to the vibrationally resolved photoelectron spectrum of HCl+ it is apparent that a single superexcited state acts as a gateway to autoionization and dissociation into H+Cl*(4s). Potential reconstruction of the superexcited state to autoionization was undertaken and from a comparison of different autoionization models it appears most likely that the gateway state is a purely repulsive and low-n Rydberg state with a (4Pi) ion core.     

Ground vs. excited state electron transfer: adsorbed monolayers and trimers in solution

Transient emission spectroscopy has been used to probe the rate of photoinduced electron transfer between metal centres within a novel trimeric complex [[Os(bpy)2(bpe)2][Os(bpy)2Cl]2]4+, where bpy is 2,2'-bipyridyl and bpe is trans-1,2-bis-(4-pyridyl)ethylene. Transient emission experiments on the trimer, and on [Os(bpy)2 (bpe)2]2+ in which the [Os(bpy)2 Cl]+ quenching moieties are absent, reveal that the rate of photoinduced electron transfer (PET) across the bpe bridge is 1.3 +/- 0.1 x 10(8) s(-1). Investigations into the driving forces for oxidation and reduction of the electronically excited state within the trimer indicate that quenching of the [Os(bpy)2 (bpe)2]2+ centre within the trimer involves electron transfer from the [bpe Os(bpy)2 Cl]+ centres to the electronically excited state with a driving force of -0.3 eV. Monolayers of the complex, [Os(bpy)2 bpe pyridine]2+, have been formed by spontaneous adsorption onto platinum microelectrodes and used to probe the dynamics of electron transfer across the trans-1,2-bis-(4-pyridyl)ethylene bridge in the ground state. These monolayers are stable and exhibit well defined voltammetric responses for the Os2+/3+ redox reaction. Cyclic voltammograms recorded at high scan rates can be accurately modelled according to a non-adiabatic electron transfer model based on the Marcus theory using a standard heterogeneous electron transfer rate constant, k(o), of 3.1 +/- 0.2 x 10(4) s(-1) and a reorganization energy of 0.4 +/- 0.1 eV. This rate constant is a factor of approximately two orders of magnitude smaller than that found for photoinduced electron transfer across the same bpe bridge for identical driving forces. This significant difference is interpreted in terms of both the nature of the orbitals involved in electrochemically and optically driven electron transfer, as well as the strength of electronic coupling between two molecular components as opposed to a molecular component and a metal electrode.     

Radiative relaxation quantum yields for synthetic eumelanin

We report absolute values for the radiative relaxation quantum yield of synthetic eumelanin as a function of excitation energy. These values were determined by correcting for pump beam attenuation and emission reabsorption in both eumelanin samples and fluorescein standards over a large range of concentrations. Our results confirm that eumelanins are capable of dissipating >99.9% of absorbed UV and visible radiation through nonradiative means. Furthermore, we have found that the radiative quantum yield of synthetic eumelanin is excitation energy dependent. This observation is supported by corrected emission spectra, which also show a clear dependence of both peak position and peak width on excitation energy. Our findings indicate that photoluminescence emission in eumelanins is derived from ensembles of small chemically distinct oligomeric units that can be selectively pumped. This hypothesis lends support to the theory that the basic structural unit of eumelanin is oligomeric rather than heteropolymeric.